1. Field of the Invention
The present invention relates to a microminiaturemicrominiature power converter, such as a DC-DC converter, composed of a semiconductor element (an electronic component) such as a semiconductor integrated circuit (hereinafter referred to as an IC) and passive components such as a coil, a capacitor, and a resistor.
2. Description of the Related Art
Recently, electronic information equipment, various types of mobile electronic information equipment in particular, have become remarkably widespread. Most of these types of electronic information equipment have batteries for power supply and contain a power converter such as a DC-DC converter. The power converter is generally constructed as a hybrid power supply module in which discrete parts of active components such as switching elements, rectifiers, and control ICs, and passive components such as magnetic parts, capacitors, and resistors, are packaged on a ceramic board or a plastic printed circuit board.
With the current demand for a reduction in size, thickness and weight of various types of electronic information equipment including the mobile ones, the need for built-in power converters having a reduced size, thickness and weight is correspondingly urgent. Miniaturization of the hybrid power supply module has been achieved by an MCM (multi chip module) technique and by a technique in which laminated ceramic parts are used. However, since discrete parts are packaged and arranged on one and the same substrate, the reduction possible in packaging area of the power supply module is limited. Particularly since a magnetic part, such as an inductor or a transformer, has a very large volume as compared with an integrated circuit, the size of the magnetic part constitutes the most critical limitation restricting the reduction in size and thickness of electronic equipment.
For reduction in size and thickness of a magnetic part, two approaches are conceivable. In one approach, the magnetic part is made as small and thin as possible and packaged on a planar board; in the second approach, the magnetic part is made in a configuration of a thin film and mounted on a silicon substrate. An example has been reported recently in which a thin micro-magnetic element (coil, transformer) is mounted on a semiconductor substrate by applying semiconductor technology. In particular, a planar magnetic part (a thin inductor) formed by using a thin film technology has been disclosed (see, for example, Japanese Unexamined Patent Application Publication No. 2001-196542) in which a thin film coil is sandwiched by a magnetic substrate and a ferrite substrate, and formed on a surface of a semiconductor substrate in which semiconductor parts, such as a switching element and a control circuit, are formed.
The foregoing structure has made it possible to reduce the thickness and packaging area of the magnetic element. This structure, however, requires a vacuum process, which increases manufacturing costs. In addition, it needs extensive steps for laminating a multiple of magnetic films and insulator films, especially when use at a high electric current is contemplated, which significantly increases costs.
A type of a planar magnetic element has been disclosed (see, for example, Japanese Unexamined Patent Application Publication No. 2002-233140 (FIG. 1)) in which a resin material mixed with magnetic fine particles is filled in the gap of a spiral coil conductor, and ferrite substrates are put on upper and lower surfaces of the conductor.
In this method, since the inductance of the coil conductor is approximately proportional to the number of turns of the spiral, it is necessary to increase the number of turns for achieving a large inductance. When the number of turns is increased without increasing the packaging area, the cross-sectional area of the coil conductor must be decreased. That is, in order to obtain a large inductance, it is necessary to decrease the cross-sectional area of the coil conductor and to elongate the conductor wire length. The decreased cross-sectional area of the coil conductor and the elongated conductor wire length bring about an increase in DC resistance of the coil conductor and an increase in power loss.
In order to solve this problem, a thin magnetic element has been disclosed (see, for example, Japanese Unexamined Patent Application Publication No. 2004-274004 and corresponding U.S. Pat. No. 6,930,584 and Chinese Patent Application Publication No. CN 1525631 A) that comprises a magnetic insulating substrate, and a solenoid coil conductor in which are connected a first conductor formed on a first principal plane of the magnetic insulating substrate, a second conductor formed on a second principal plane of the magnetic insulating substrate, and a connection conductor formed in a through-hole passing through the magnetic insulating substrate.
The foregoing structure provides a microminiaturemicrominiature, thin power converter in which terminals for connection to a semiconductor element and a circuit board can be formed at the same time in the process for forming the through-holes and the coil conductors on the magnetic insulating substrate. Then, the IC chip only needs to be mounted on the magnetic insulating substrate for forming the coil, thus eliminating necessity for a separate packaging substrate.
The microminiature power converter disclosed in Japanese Unexamined Patent Application Publication No. 2004-274004 features a power converter comprising a thin magnetic induction element that includes a through-hole formed in a magnetic insulating substrate, coil conductors electrically connected through the through-hole and formed on the first principal plane and the second principal plane, and further electrodes (connection terminals) formed on the first principal plane for electrical connection to the semiconductor element, and other electrodes (packaging terminals) formed on the second principal plane for electrical connection to a printed circuit board that is used in actual operation. The proposed constitution provides a power converter with a reduced thickness while limiting the number of parts composing the converter to the minimum. This microminiature power converter is described in the following.
FIGS. 15(a) and 15(b) show a microminiature power converter of the prior art, in which FIG. 15(a) is a sectional view of a thin magnetic induction element on which is mounted an IC chip, and FIG. 15(b) is a plan view of the thin magnetic induction element seen from the side of the first principal plane (front surface). FIG. 15(a) is a sectional view taken along the line Y-Y in FIG. 15(b). In FIG. 15(b), the IC chip 80 is indicated by a dotted line.
The thin magnetic induction element 300 of the prior art includes a solenoid coil formed in a central region of a ferrite substrate 86 and electrodes 82, 88 formed in a peripheral region. The solenoid coil is composed of a coil conductor 84 (including connecting conductors) formed on the first principal plane and the second principal plane of the ferrite substrate 86 and in the through-holes 85. The electrode 82 is formed on the first principal plane and the electrode 88 is formed on the second principal plane, both electrodes being connected by a connection conductor 83a formed in the through-hole 83.
An IC chip 80 is bonded to the electrode 82 formed on the first principal plane through a stud bump 81. An under fill resin 89 is filled in the space between the IC chip 80 and the ferrite substrate 86. The coil conductor 84 formed on the second principal plane is covered by a protective film 87.
A lamination substrate, though not a microminiature power converter, has been disclosed (see, for example, Japanese Unexamined Patent Application Publication No. 2005-183890) in which a coil is internally formed of a coil conductor and a magnetic material, external electrodes for mounting a circuit component are formed on the upper surface, and a wiring pattern is internally formed for wiring the external electrodes and the coil. The lamination substrate has enhanced freedom of arrangement of circuit component to be mounted.
The structure disclosed in Japanese Unexamined Patent Application Publication No. 2004-274004 has, as shown in FIGS. 15(a) and 15(b) herein, bonding electrodes (electrodes 82) in the peripheral region of a thin magnetic induction element for bonding to IC chip 80, i.e., a semiconductor element. Since the coil of the thin magnetic induction element having the configuration of a solenoid coil must be located in the central region of the magnetic induction element, the bonding electrodes for bonding with a semiconductor element such as IC chip 80 are necessarily located in the peripheral region of the thin magnetic induction element.
Among the characteristics of a thin magnetic induction element, the inductance value largely depends on the size of the magnetic induction element. As such, element size primarily affects feasibility of necessary characteristics.
Concerning the size of a semiconductor element, however, the size can be restricted of course by the characteristics, but may be reduced in large extent depending on desired performance in some cases.
Thus, there is no intrinsic correlation between the size of a semiconductor element and the size of a thin magnetic induction element. Nevertheless, a semiconductor element is mounted on the magnetic induction element in planar configuration in the foregoing structure and electrodes 82 are arranged in the peripheral region. As such, the problem arises that the size of the semiconductor element is eventually restricted by the size of the thin magnetic induction element.
Thus, even though a semiconductor element could be minimized, if a thin magnetic induction element is not minimized, the semiconductor element must have a large size, inhibiting cost reduction.
Further, since the electrodes 82 are arranged in the peripheral region, it is difficult to package two or more semiconductor elements. Thus, the restriction arises that the number of semiconductor elements that can be packaged on one thin magnetic induction element is limited to only one. This restriction has inhibited multi-functionality of the device.
Although the lamination substrate of Japanese Unexamined Patent Application Publication No. 2005-183890 has the freedom of arrangement of parts connecting to the internally formed coil (inductor), it also has a complicated structure, which leaves the problems of cost and micro-miniaturization.
It is therefore an object of the present invention to solve the above problems and provide a microminiature power converter in which components including a semiconductor element smaller than a thin magnetic induction element can be packaged in a planar arrangement at a predetermined place on the thin magnetic induction element without restriction on positioning of the electrodes formed in the peripheral region of the thin magnetic induction element.